Photographic positive-producing light sensitive element and process



Aug. 21, 1951 E. H. LAND 2,565,376

PHOTOGRAPHIC POSITIVE-PRODUCING LIGHT SENSITIVE ELEMENT AND PROCESS Filed Feb. 19. 1947 Coniciner {Tronspdrenf Base Layer r I /7\ [5 A Posiiive Pho'losensi'llve Layer Opaque Base Layer FIG. I

[4 Trunspuren? Separuiing 2O /////9 Layer l6 l8 Coniciner FIG. 2

Patented Aug. 21, 951

PHOTOGRAPHIC PO SITIV E-PRODUCIN G LIGHT SENSITIVE ELEMENT PROCESS Edwin H. Land, Cambrid Polaroid Corporation,

poration of Delaware Application February 19, 1947, Serial No. 729,579

.12 Claims.

This invention relates to photography and more particularly to novel photographic products and processes.

It is a particular object of the present invention to provide a novel photographic process for obtaining a positive image in a single processing operation after exposure of a silver halide photosensitive layer to create therein a latent negative image.

Another object of the invention is to provide novel products particularly useful with a process of the above type.

These and other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the product possessing the features, properties and the relation of components, and the process involving the several steps and'the relation and the order of one or more of such steps with respect to each of the others, which are exemplified in the following detailed disclosure, and the scope of the application of which will be indicated in the claims. l 1

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawing wherein:

. Figure 1 is a diagrammatic sectional view of a preferred form of film unit of the present invention; and

Fig. 2 is a diagrammatic sectional view of another form of the. invention.

This invention relates generally to a one-treatment photographic process for obtaining a fixed positive image of a latent negative image created by the exposure of a silver halide photo-sensitive layer to actinic lightmodified by a subject image. In a preferred form of the invention a silver halide photosensitive layer, carried by a translucent or transparent. base, is exposed to actinic light to create therein a latent negative image. This layer is. next formed into a lamination with an unexposed silver halide photosensitive layer, carried on an opaque base, by preferably spreading between said layers a viscous solution of a developer and a silver halide solvent. The developer develops the latent negative image, and also permeates the positive unexposed photosensitive layer. The positive layer. is then exposed to actinic light passing through the developed negative photosensitive layer to create a latent positive image'in the positive photosensitive layer; The developer then develops the latentpositive imageand the silver halide sol vent dissolves the unexposed silver halide from AND ge, Mass., assignor to Cambridge, Mass., a corthe positive layer. In one preferred modification of the invention, the light used to create the positive latent image also makes developable the previously unexposed grains in the negative layer and these grains are developed by the developer in contact therewith to convert the negative layer to an opaque layer, thereby constituting an automatic exposure control for the positive latent image.

Referring now to Figure 1 there is shown a diagrammatic sectional view of a preferred film unit of the present invention. In this form of the invention there is provided a base layer Ill which is preferably a transparent cellulosic film base material. On this base there is a layer l2 of a photosensitive material which is preferably a silver halide and preferably a fast emulsion. There is next provided a container l4, having therein a liquid composition which preferably includes a developer and a silver halide solvent. There is also provided a second photosensitive layer I6 which is preferably a silver halide and preferably is slower than the emulsion l2. This second photosensitive layer It is preferably carried by an opaque paper base 18.

In a preferred form of the invention the above film unit is formed of the following materials. The layers Ill and I2 comprise a sheet of a fast photographic film such as that sold by Ansco under the trade name of Triple S Pan, while the layers It and I8 comprise a sheet of copy paper emulsion such as that sold by Kodak under the trade name of Kodabromide F3.

The container I4 is preferably formed of a paper-backed metal foil lined with an alkaliinert plastic such as polyvinyl butyral, this .foil being folded along a medial line and the overlapping edges being heat sealed together, the edge seal opposite the fold being preferably weaker than the seal at the ends of the container. The liquid in the container I4 is preferably a viscous alkaline aqueous solution of a developer, a silver halide solvent, a film-forming material, and other materials capable of assisting in the processing of the film unit. A preferred liquid composition is prepared by mixing the following ingredients:

Example 1 Water cc 1860 Sodium carboxymethyl cellulose (medium viscosity) grams 93 Sodium sulfite do '78 Sodium hydroxide do- 74.6 Sodium thiosulfate do 14.5 Citric acid do 38.5 Hydroquinone do 52 In a preferred use of the film unit shown in Fig. 1, it is positioned in a camera with the various layers in the position shown. The negative photosensitive layer I2 is then exposed to aotinic light passing through the base layer H3. Due to the fact that the negative photosensitive layer :2 is much faster than the positive photosensitive layer !6, the small amount of light used in exposing layer l2 does not have any appreciable effect upon layer It. It is equally possible, but less desirable, from considerations of camera simplicity, to separate layers l 2 and is before exposure, or to expose layer 12 before it is brought into juxtaposition with layer it. After the exposure of negative layer l2, the two layers, with the container i4 therebetween, are passed between a pair of pressure rollers to cause a translational compression to be applied to the assembly. This releases the liquid from the container M and translates this bulk of liquid between the two photosensitive layers [2 and I8. Ihis movement of the liquid leaves a predetermined film of the liquid between these layers, thus laminating them together. After a time suiiicient for the negative to develop, at least partially, to the point where considerable density is apparent, the assembled lamination is exposed to uniform light, such as by being taken from the camera into daylight. This may be accomplished by so positioning the discharge opening from the camera with relation to the pressure-applying means, and speed of travel of the film unit, that the development of the negative is substantially complete when the unit is discharged. This light passes through transparent base layer Ill and is modified, p ing through negative layer I2, in accordance with the density of the developing negative i' The light thus differentially exposes the tive layer It to create therein a latent posi ve image which is rapidly developed due to the fact that the positive layer has been permeated by the developer in the liquid composition. Meanwhile the second exposure of the negative layer i2 makes the previously unexposed silver halide therein developable and these grains are subsequently developed to convert the whole of the negative layer into a black substantially opaque layer.

From the time that the liquid composition spread between the two layers, the silver halide solvent therein dissolves undeveloped silver hal ide from both layers and transfers the dissolved silver, in the form of silver complexes, to the layer of liquid composition. These complexes may remain in solution but are preferably reduced by the developer in the liquid layer to particles of silver, thereby freeing the thiosulfate ion for further dissolution of silver halide. Since most of the silver halide in negative layer i2 is developed to metallic silver, in situ, the silver halide solvent primarily dissolves the silver halide which is undeveloped in the positive layer thereby fixing the positive image. Due to the fact that the liquid composition preferably con tains a film-forming material (sodium carboxymethyl cellulose in Example 1), this film-forming material creates a dimensionally stable film which can be separated from the positive layer after the completion of the above processing. The surface of positive layer it is preferably less adherent to the formed film than is the surface of layer I2 so that when these layers are separated the formed film adheres to the negative layer. One way of accomplishing this is to coat the positive layer 16 with a soap solution.

The above process and film unit thus make possible the creation of a stable positive image from a latent negative image by one mechanical treatment of the film unit.

A less preferred form of film unit capable of accomplishing such a one-treatment processing is shown in Fig. 2 where like numbers correspond to like elements of Fig. 1. In Fig. 2 there is provided, in addition to the negative" layer Ill-I2 and the positive layer Iii-l 8, a separating layer 28. In this case there are preferably provided two containers, M and 22. In this case the liquid in container l4 may be like the liquid of Example 1, but preferably has no silver halide solvent, while the liquid in container 22 may be like Example 1. The separating layer 21] is preferably made of a relatively impermeable transparent sheeting to which the film-forming material in container 22 will adhere more strongly than to the surface of positive layer Hi. This layer 20 may comprise cellulose acetate while the other elements of the film unit may be made of materials stated as being preferable in the discussion of Fig. 1 above.

After exposure of the negative layer 12 in the film unit of Fig. 2, it may be processed in the same manner as that discussed in connection with Fig. 1 above, being preferably passed between a pair of pressure members such as pressure rolls which apply a translatory compression to the film unit, causing the release of the liquids from containers [4 and 22 and the spreading of the released liquid. The films of liquid laminate the various layers together and the two liquids permeate their respective photosensitive. layers. The liquid from container I4 develops the latent negative image and, upon the second exposure, completes development of the negative. layer to uniform density. The liquid from container 22 develops and fixes the latentv positive image created in the positive layer during the second exposure of the film unit.

In those cases where it is desired to use silver halide emulsions in both the positive and negative layers which have substantially the same speed and it is desirable to have these layers superimposed during the exposure of the negative layer, this result is preferably accomplished by employing a dyed light barrier between the two layers. This dyed layer can be a nonhalation layer in Fig. 1 or can be layer 20 in Fig. 2. In such a case the dye is so chosen that it can be bleached by the spread liquid in contact therewith, so as to convert the dyed layer to a transparent layer.

Layer 20 can be slowly permeable so that elements of one liquid can be combined with elements of the other liquid after a. predetermined time delay to create additional functions in the image-forming processes or to aid in stabilizing the negative or positive images.

The liquid in container M of. Fig. 2 may contain a color-forming developer which is useful, in some cases, to increase the opacity of the negative where development occurs, and which also may be useful in increasing the total opacity of layer [2 when this layer is allowed to develop completely during the exposure of the positive" layer It. If the use of a color-forming developer is desired in Fig. 1 it is preferably one that couples with a color former, included only in layer [2, thereby giving a dye image only in layer 12.

In the modifications of the invention shown in Figs. 1 and 2 above, the base layer [B may include a sulfide or selenide which has. the function of aiding the fixing of the positive layer I 6 by speeding up the precipitation ofsilver from the silver complex. This sulfide or selenide may also be included in the liquid composition which con-' tacts the positive layer 16 or. in Fig. 2, it may be coated on the lower surface of separating layer 20.

While the preferred film unit of the present invention comprises the two photosensitive layers suitably joined together witha container positioned for release of its liquid between these two layers, other modifications of the invention are equally possible. For examplefthe negative photosensitive layer may be in a roll, the positive photosensitive layer may also be in a roll; and the containers may be fed between these two rolls as they are brought together afterexposure of the negative photosensitive layer. It is also possible to include the containers in one of these rolls, these containers being predeterminedly spaced on one of the layers. Ina preferred form of the invention the film unit is designed so that the negative photosensitive layer is exposed through its base layer, this form permitting a completely sealed film unit from which the processing liquid cannot escape.

While the preferred forms of the product and process of the invention have been shown in Figs. 1 and 2, they may be considerably modified. For example, the container 22 in the film unit of Fig. 2 may be positioned at the right-hand side of the film so that this container will not be broken and the fluid spread therefrom at the same time that the liquid is released and spread from container M. This feature of the invention is of advantage in those cases where the development of the negative takes considerable time and it is not desired to have the solution of the developer and/or silver halide solvent in contact with the positive photosensitive layer during this time. In this case the fluid from the container 22 may be released just prior to. during, or after the exposure of the positive layer IE to actinic light modified by the developed negative.

It is also possible to provide still another container, in either of Figs. 1 and 2, which contains a silver halide solvent so that the contents of this container may be released subsequent to the development of the positive image to thereby fix the positive image. It is possible also to have the containers fed from magazines therefor and to have the positive and negative layers fed from separate rolls, although this form of the invention is less preferred since it requires a considerably more complicated apparatus to accomplish the desired processing. While it is preferred that the liquid reagent in the containers be released between two layers and spread therebetween so that the liquid in no case contacts any portion of the photographic apparatus, it is possible to spread the released liquid against an inert nonadhering surface such as a chromium wheel or plate forming a part of the apparatus.

While the above invention has been described in terms of creating a latent negative image, developing the latent negative image and then exposing through this negative to create a latent positive image, it may be advantageously modified as set forth below. In this case the negative photosensitive layer I 2, of Fig. 2 for example, may be exposed for a very short time to the subject image with the lens out of focus to create.

anunsharp mask latent image in the negative photosensitive layer, this unsharp mask image being then developed, and-a dyed layer between the negative and positive layers being bleached. This processing is preferably accomplished 'by releasing a processingliquidfrom container l4 and spreading it; in contact with layer l2. This initial exposure may be made behind an unfocused lens and the film unit may be then moved behind a focused lens, the liquid being released as the. result ofthis movement. The film unit may also be processed without movement by providing a pressure member that moves across the focal plane of the camera.

The other photosensitive layer I6 is then exposed to a sharply focused subject image, the light passing through the unsharp mask, thus creating a sharp latent negative image in layer I6. The developer develops this sharp latent negative image in layer i 6 and the silver halide solvent in the liquid forms soluble silver complexes with-the 'unexposed silver halide, transfers these complexes to the layer of, liquid where these complexes are converted to black particles of silver by the developer, preferably with the assistance of a sulfide or selenide. When the sulfide or selenide is included on the lower surface of layer 20, the positive image is produced on this surface and in the layer of liquid which has preferably formed a dimensionally stable film bythis stage of the process. This film adheres to transparent layer 25!, thereby producing a positive image on the layer 26 which can be separated from the two negative layers. This form of the process of the invention has the additional advantage that it constitutes an automatic exposure control for the sharp latent negative image since the average amount of light forming this sharp latent negative image is reduced as a direct function of the intensity of the light emanating from the subject image. Thus if the subject image is bright, a large amount of the light used in forming the sharp latent negative is masked out, whereas if the subject image is dull, a relatively small amount of light emanates therefrom, the mask is much more transparent, and a relatively smallperoentage of the light used to form the sharp latent negative image is masked out. i

In connection with the above masking technique where two latent negative images are formed, one being sharp and the other being fuzzy, the film unit of Fig. 2 may be considerably modified by having the photosensitive layer 16 carried by transparent layer 20, with the second container 22 being positioned for discharge of its contained liquid between layers [6 and I8. In this case the positive image from the sharp latent negative image in layer i 6 may be formed on the surface of layer Hi.

This feature of the invention can also be accomplished by modifying the film unit to have sequential releasing of the liquid from the containers rather than simultaneous release of the liquids. Y 1

Although the various elements of the liquid may be included in solid form in the film unit instead of being in solution, and such a modification of the invention is included in its scope,

this modification of the invention is less pre- What is claimedis:

'1. A photographicproduct comprising a transparent support; a silver halide photosensitive layer carried on said support; a second support; a silver halide photosensitive layer carried on said second support; said photosensitive l'ayersbeing adapted for successive exposure with the first photosensitive layer being exposed substantially without effect upon said second photosensitive layer; and rupturable container means holding processing materials including at least a liquid solvent for a photographic developer, an organic film-forming material soluble in said liquid, and a silver halide developer; said supports, said photosensitive layers and. said container means being held together to perunit at least a portion of said supports and photosensitive layers to be superposed with said pho tosensitive layers facing each other and with said container means located between said supports and so positioned as to be capable of being ruptured and without removal of its ruptured portion of releasing its liquid content between two layers of said product to at least partially permeate the superposed photosensitive layers; said product also having positioned therein as another processing material a silver halide solvent soluble in said liquid and rendered effective to form a silver complex with unexposed silver halide in at least one of said photosensitive layers upon release of said liquid from said container means; said processing materials being capable of developing exposed silver halide in each photosensitive layer and of forming between said photosensitive layers a transparent film permeable to said liquid and as a result of development of a latent image in at least said second photosensitive layer causing a differential dispersion throughout said second photosensitive layer of a silver halide complex which is transportable by imbibition to said transparent film for reduction to metallic silver.

2. A photographic product of the. character defined in claim 1 wherein said first-mentioned photosensitive layer comprises a relatively fast silver halide emulsion and wherein said secondmentioned photosensitive layer comprises a relatively slow silver halide emulsion.

3. A photographic product as defined in claim 1 wherein the first-mentioned photosensitive layer is relatively fast and the second-mentioned photosensitive layer is relatively slow and wherein said container means comprises a runturable container located between said photosensitive layers and within which all said processing materials are contained.

4. A photographic product of the character set forth in claim 1 having as a first-mentioned photosensitive layer a relatively fast emulsion adapted to have a negative latent image formed therein and as a second photosensitive layer a relatively slow emulsion adapted to have a latent image which is a positive of said negative latent image formed therein, a substantially transparent impermeable layer positioned between said photosensitive layers, and container means comprising an individual container for each hotosensitive layer ositioned between said transparent impermeable layer and the sup port for the photosensitive layer with which said individual container is associated.

5. A photographic product of the character set forth in claim 1 having as a first-mentioned photosensitive layer a relatively fast emulsion adapted to have a negative. latent image formed therein andfas; a second photosensitive layer a relatively slow'emulsion adapted to have a latent image which is a positive of said negative latent image formed therein, a substantially transparent impermeable layer positioned between said photosensitive layers, and container means comprising an individual container for each photosensitive layer positioned between said transparent impermeable layer and the support for the photosensitive layer with which said individual container is associated, said silver halide solvent being included in said container associated with the relatively slow emulsion.

6, A hotographi process which comprises differentially exposing a photosensitive silver halide layer, mounted on a transparent support, throu sa supp rt and forming a atent im ge therein wh le restrictin the. action o light employed. for said exposure substantially to said phot s nsi ive layer; spreading liquid c mp tion containing photographic processing material in a solvent between said first-mentioned photosensitive layer and a second photosensitive layer of silver halide mounted on an opaque support and arranged in superposed relation to said first photosensitive layer whereby said supports are outermost; said processing materials comprising a silver halide developer, a silver halide solvent and an organic film-forming material; permeating said processing composition, include ing at least said developer, into said first-mentioned layer and developing latent image formed by said eXposure'to silver; said processing com! position, including at least said developer and said silver halide solvent, being also permeated into said secondementioned and unexposed layer at least while development of said first-mentioned layer is, proceeding and said filmformin g material contained in said composition providing a transparent permeable film between said photosensitive layers at least adjacent the surface of the second-mentioned photosensitive layer; after development of said first-mentioned layer differentially exposing said second-mentioned layer by directing light actinic thereto through said first-mentioned layer including said developed image and onto said second-mentioned layer; developing the latent image formed in said second-mentioned photosensitive layer with said liquid processing composition which is permeated into said second-mentioned layer and as a result of said development forming a differential dispersion throughout said second photosensitive layer of a soluble silver complex and transporting complex in solution by imbibition to said film formed by said film-forming material; and reducing said complex in said film to metallic silver.

7. A photographic process as set forth in claim 6 including the step carried out following development in bothsaid photosensitive layers of separating said layers from said superposed relation with respect to each other and with respect to saidfilm provided by said film-forming materi l between said layers.

8. A photographic p ce of h character fined in claim 6 wherein said liquid composition is confined within a rupturable container located between and in contact with both said photosensitive layers and including the step of rupturing said container whereby to release the liquid content thereoffor spreading between said layers.

9. A photographic process of the character defined in claim 6 wherein said liquid composition is confined within a rupturable container located between and in contact with both said photosensitive layers and including the step of rupturing said container whereby to release the liquid content thereof for spreading between said layers, and wherein said first-mentioned photosensitive layer comprises a relatively fast silver halide emulsion and said second-mentioned photosensitive layer comprises a relatively slow silver halide emulsion.

10. A photographic process of the character defined in claim 6 wherein said first and second photosensitive layers are positioned in superposed relation on opposite sides of a separating layer and wherein an individual rupturable container adapted to contain said liquid composition is located on each side of said separating layer and between said separating layer and the support for the photosensitive layer adapted to be processed by the liquid content of said container, said process including the step of rupturing said containers whereby to release the liquid content thereof for spreading between and in contact with each of said photosensitive layers.

11. A photographic process of the character defined in claim 6 wherein a separating layer is interposed between said photosensitive layers and a rupturable container for liquid processing composition, including at least a photographic developer and an organic film-forming material, is located between said separating layer and the support for said first photosensitive layer and wherein a rupturable container for liquid processing composition, including at least a photographic developer, an organic film-forming material and a silver halide solvent, is positioned between said separating layer and said support for said second photosensitive layer, said process including the step of rupturing said containers whereby to release the liquid content thereof for spreading in contact with each said photosensitive layer, said first photosensitive layer comprising a relatively fast silver halide emulsion and said second photosensitive layer comprising a relatively slow silver halide emulsion.

12. A photographic process as set forth in '10 claim 6 including the steps of exposing said firstmentioned photosensitive layer, substantially without eiiect upon said second-mentioned pho=- tosensitive layer, to light from a subject directed onto said first photosensitive layer by a lens which is in poor focus with said subject whereby to form an unsharp negative latent image in said first-mentioned photosensitive layer, after development of the negative latent image in said first-mentioned photosensitive layer exposing said second photosensitive layer to light from said subject directed by a lens which is sharply focused on said subject through said first photosensitive layer, including the developed negative image therein, and onto said second photosensitive layer whereby to form a relatively sharp negative latent image in said second photosensitive layer, transporting said dispersion of said soluble silver complex, formed in said second photosensitive layer, by imbibition to said film provided by said film-forming material to form therein a differential distribution of said silver complex, and reducing said complex in said film to metallic silver to provide a positive image in silver of the negative latent image formed in said second photosensitive layer.

EDWIN H. LAND.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 742,405 Eichengrun Oct. 27, 1903 1,285,015 Browning Nov. 19, 1918 1,303,635 Capstaif May 13, 1919 1,841,653 van der Grinten Jan. 19, 1932 1,956,230 Schmidt Apr. 24, 1934 2,315,966 Knott Apr. 6, 1943 2,352,014 Rott June 20, 1944 FOREIGN PATENTS Number Country Date 369,146 Germany Feb. 15, 1923 370,821 Germany Mar. 8, 1923 716,428 France May 1, 1931 879,995 France Dec. 10, 1942 

1. A PHOTOGRAPHIC PRODUCT COMPRISING A TRANSPARENT SUPPORT; A SILVER HALIDE PHOTOSENSITIVE LAYER CARRIED ON SAID SUPPORT; A SECOND SUPPORT; A SILVER HALIDE PHOTOSENSITIVE LAYER CARRIED ON SAID SECOND SUPPORT; SAID PHOTOSENSITIVE LAYERS BEING ADAPTED FOR SUCCESSIVE EXPOSURE WITH THE FIRST PHOTOSENSITIVE LAYER BEING EXPOSED SUBSTANTIALLY WITHOUT EFFECT UPON SAID SECOND PHOTOSENSITIVE LAYER; AND RUPTURABLE CONTAINER MEANS HOLDING PROCESSING MATERIALS INCLUDING AT LEAST A LIQUID SOLVENT FOR A PHOTOGRAPHIC DEVELOPER, AN ORGANIC FILM-FORMING MATERIAL SOLUBLE IN SAID LIQUID, AND A SILVER HALIDE DEVELOPER; SAID SUPPORTS, SAID PHOTOSENSITIVE LAYERS AND SAID CONTAINER MEANS BEING HELD TOGETHER TO PERMIT AT LEAST A PORTION OF SAID SUPPORTS AND PHOTOSENSITIVE LAYERS TO BE SUPERPOSED WITH SAID PHOTOSENSITIVE LAYERS FACING EACH OTHER AND WITH SAID CONTAINER MEANS LOCATED BETWEEN SAID SUPPORTS AND SO POSITIONED AS TO BE CAPBALE OF BEING RUPTURED AND WITHOUT REMOVAL OF ITS RUPTURED PORTION OF RELEASING ITS LIQUID CONTENT BETWEEN TWO LAYERS OF SAID PRODUCT TO AT LEAST PARTIALLY PERMEATE THE SUPERPOSED PHOTOSENSITIVE LAYERS; SAID PRODUCT ALSO HAVING POSITIONED THEREIN AS ANOTHER PROCESSING MATERIAL A SILVER HALIDE SOLVENT SOLUBLE IN SAID LIQUID AND RENDERED EFFECTIVE TO FORM A SILVER COMPLEX WITH UNEXPOSED SILVER HALIDE IN AT LEAST ONE OF SAID PHOTOSENSITIVE LAYERS UPON RELEASE OF SAID LIQUID AND RENDERED EFFECTIVE TAINER MEANS; SAID PROCESSING MATERIALS BEING CAPABLE OF DEVELOPING EXPOSED SILVER HALIDE IN EACH PHOTOSENSITIVE LAYER AND OF FORMING BETWEEN SAID PHOTOSENSITIVE LAYERS A TRANSPARENT FILM PERMEABLE TO SAID LIQUID AND AS A RESULT OF DEVELOPMENT OF A LATENT IMAGE IN AT LEAST SAID SECOND PHOTOSENSITIVE LAYER CAUSING A DIFFERENTIAL DISPERSION THROUGHOUT SAID SECOND PHOTOSENSITIVE LAYER OF A SILVER HALIDE COMPLEX WHICH IS TRANSPORTABLE BY IMBIBIBTION TO SAID TRANSPARENT FILM FOR REDUCTION TO METALLIC SILVER. 